1. Field of the Invention
The present invention relates to a mold used mainly for press molding of a highly precise optical element and a method for molding optical elements.
2. Description of the Prior Art
Recently, optical elements have tended to have a nonspherical surface with which both simplification of lens configuration and weight reduction of the lens part in an optical apparatus can be simultaneously achieved. U.S. Pat. No. 4,481,023 proposes a method of manufacturing a highly precise glass product with a low cost, by which a highly precise optical glass element can be obtained by shortening the time required for a pressurization cycle. The method proposed is outlined as follows. Using a glass preform having a shape similar to its final product, a mold and the glass preform are heated separately from each other, a pressing operation is initiated at a temperature corresponding to a glass viscosity of 10.sup.8 poise or more and 10.sup.12 poise or less, respectively, for glass and metal molds, and the pressing operation is finished at a temperature corresponding to a glass viscosity less than 10.sup.13 poise. Then the glass is taken out.
Japanese Patent Publication No. 56-378/1981 discloses a molding method for molding a raw lens material in which the temperature of a mold is kept at a value equal to or greater than the transition point and the softening point of a molded glass, a glass having flow properties is poured into the mold to be press molded. This condition is held for 20 seconds or more until the temperature distribution of the glass becomes uniform.
However, in the prior art examples mentioned above, a glass and a mold have been separately heated and then the glass has been supplied to inside the mold to be press molded, so that in the course of press molding the temperature is very liable to be non-uniform, requiring the press to be held for a certain time. Since a heating section is provided on the entire peripheral portion of the mold, rapid cooling is impossible. Accordingly, this has negatively affected cycle time, with the result that a low-cost lens cannot be manufactured.
As disclosed in Japanese Patent laid open application No. 61-26528/1986, a method has been known by which, after press molding, cooling is performed by using many molds and sequentially transferring them to a temperature-decline type slow cooling chamber. Also, the cooling rate after press molding is a very important parameter which affects lens performance. Japanese Patent laid open application No. 61-53126/1986 discloses a cooling at a constant rate of 0.9.degree. C./sec. or less without pressurization.
Thus, the temperature control from press molding to cooling has been known to be important in order to obtain a highly precise transferred surface. Slowing the cooling rate causes cycle time to become long, making difficult the supply of a low-cost optical element. Also, an optical element with a substantially different wall thickness between its center portion and peripheral portion is liable to develop a temperature difference, making it difficult to obtain a desired lens performance.
Further, U.S. Pat. No. 2,292,917 discloses a mold through which a thick-walled lens part is liable to cool and which uses two different materials in thermal conduction in order to make it difficult to develop a temperature difference between the center portion and the peripheral portion of an optical element. However, the mold is complex in structure, making it difficult for the mold to be located between a pair of heater blocks and transferred. Also, U.S. patent application Ser. No. 07/198,929 (filed on May 24, 1988) discloses a method by which a thin-walled lens portion is heated so as not to develop a temperature difference in the plastic inside the cavity. However, a heating section is provided on the peripheral portion of a mold, causing the cycle time to become long.